1. Field of the Invention
The present invention relates to a molding method for fabricating a metal composite compact according to a metal powder injection molding (MIM) process by integrating two compacts of the same type, or of different types, of material.
2. Description of the Related Art
In recent years, metal powder injection molding (MIM) has been used as a method for fabricating a metal compact. According to this method, metal powder is mixed with a binder, to give fluidity, and is subjected to injection molding. Almost all the binder is removed by heating or the like from the compact in a degreasing step, and the compact is heated to a higher temperature to sinter the metal powder in a sintering step thereby to produce the desired product.
Also, a sintered metal composite compact can be fabricated by integrating a plurality of sintered compacts of the same type or different types of material using the MIM method. In such a case, as shown in FIGS. 6A and 6B, a first compact 91 fabricated in advance is inserted in a die 8. A second compact 92 of a material identical to or different from the first compact 91 is integrated with the first compact 91 and molded thereby to produce a metal composite compact. The integrated compact is later degreased and sintered to produce the aforementioned sintered metal composite compact.
However, the metal composite compact described above poses the problem that the concentration of the binder contained in the molding material is liable to increase in the boundaries of a plurality of compacts and a sound joining boundary cannot be obtained after sintering. Specifically, at the boundary surface S in FIG. 6B, a high binder concentration portion 918 is formed in the surface of the joining surface of the first compact 91, as shown in FIG. 7. On the other hand, the surface of the joining surface of the second compact 92 is also formed with a high binder concentration portion 928. The boundary is obtained in which the two high binder concentration portions 918, 928 join each other.
Specifically, the molding material used for the MIM method is a mixture of metal powder and a binder, and is fluidized by heating it to a predetermined temperature to liquefy the binder. In the fluidized state of the molding material, the binder has a higher fluidity than the metal powder.
As a result, as shown in FIG. 8, at the time of injection molding, the high binder concentration portion 951 of the molding material 95 blows out from the central portion of the flow path to the forward end and flows back around the side surface. Thus, the surface of the molded compact is formed with a layer high in both fluidity and binder concentration, which layer also remains in the boundary surfaces of the two compacts. The high binder concentration portion which first solidifies is subjected to a shearing force F by the internal flow on the side surface in contact with the die 8 along the direction of the flow. Therefore, the thickness of the high binder concentration portion on the side surface is thinner than that on the forward end portion.
In the case where the molding material is degreased while the high binder concentration portion remains on the boundary surfaces of a plurality of the compacts, a depression may be formed due to the loss of the binder. In such a case, a normal joint may not be obtained in the subsequent sintering process.
Also, according to the MIM method, a compact of a comparatively complicated shape can be fabricated. In the case where different types of material are integrated or a compact formed of a material of the same type is geometrically too complicated, however, the injection molding may not be accomplished at one time. In such a case, the two compacts are injection molded individually and integrated in a given step of the fabrication process.
According to a method in which a sintered compact is produced by integrating a plurality of parts of the same material or different types of material using the conventional MIM, a plurality of sintered compacts formed separately from each other are joined through an appropriate process such as welding. A welding step added after sintering, however, leads to an unstable quality and a higher cost due to an increased number of steps.
Another conventional method is an insert molding method such as disclosed in Japanese Unexamined Patent Publication (Kokai) NO. 3-232906, in which a first compact prepared separately is placed in a die and a second compact is molded by injection.
In this method, the first compact is required to be fabricated separately in advance, and therefore the number of steps is increased.
Also, in the case where the first compact is placed in a die, the size of the recess into which the first compact is inserted is required to be larger than the size of the compact, resulting in a lower dimensional accuracy. This is by reason of the fact that in the case where the recess and the compact have the same size, the compact is cut off or broken when inserted into the recess, leading to a higher rejection ratio.
Japanese Unexamined Patent Publication (Kokai) No. 7-90312 discloses still another method in which the die is formed with partitions and the injection molding is carried out in cavities sequentially for integration while moving the partitions.
In this case, a compact having a three-dimensional complicated shape which the MIM method is primarily intended for is very difficult to mold, and the realization of such a molding greatly complicates the die structure and the cost becomes higher.
In the case where the compact to be formed is small, it may be that the space for partitions cannot be secured or a sprue, a runner or the like cannot be arranged in the die.
Also, it is very difficult to obtain a plurality of compacts at a time without resolving the problem of a very complicated die structure and a higher cost.
Further, the provision of a multiplicity of sliding portions for moving the partitions shortens the life of the die.
Furthermore, as the result of adding the partitions, a multiplicity of sectional die surfaces are transferred to the compact and a superior surface cannot be obtained. Another problem is that a high dimensional accuracy cannot be secured due to the effect of the clearance unavoidably existing in the sliding parts of the partitions.
Japanese Unexamined Patent Publication (Kokai) No. 4-346604, on the other hand, discloses a method in which an inserted core is removed after forming a first compact and a second compact is molded by injection into the cavity formed by the core. Also, the second compact is molded by injection after the portion of the first compact to be in contact with the second compact is maintained at 20 to 70xc2x0 C.
For the temperature of the outer surface of the first compact to be maintained at 20 to 70xc2x0 C., however, it is necessary to wait until the temperature drops after injection molding in an ordinary injection molding machine, resulting in a very low productivity.
Also, the molding process using a core consumes a considerable length of time for mounting and demounting the core. Since the core is cooled once removed, it is necessary to wait until the core temperature increases to a predetermined level before continuing the injection molding, also resulting in a low productivity.
The present invention has been developed in view of the aforementioned problems, and a first object thereof is to provide a method of fabricating a metal composite compact in which the forming of a high binder concentration portion can be suppressed at the boundary surfaces of a plurality of compacts made of materials of different types or a material of the same type.
A second object of the invention is to provide a method of fabricating a metal composite compact including an integration of a plurality of compacts of the materials of different types or a material of the same type with a superior dimensional accuracy at low cost.
According to a first aspect of the invention, there is provided a method of fabricating a metal composite compact in which a first compact is molded by injection using the MIM method for injection-molding a molding material composed of a mixture of metal powder and a binder in a die, after which a second compact is molded by injection in close contact with the joining surface of the first compact thereby to integrate the first and second compacts, wherein the second compact is molded by injection in such a manner that the second molding material of the second compact is rendered to flow while being filled in the die to obtain a flow component in the direction parallel to the joining surface of the first compact.
The most noteworthy fact in the present invention is that the direction of flow of the second molding material is positively controlled when the second compact is molded by injection as described above to thereby create a direction of flow parallel to the joining surface of the first compact. This flow component parallel to the joining surface is not limited to the one flowing linearly from the beginning but also includes the one curved on a surface parallel to the joining surface.
In this aspect of the invention, after molding the first compact by injection, the second compact is molded by injecting the second molding material into a die in which the first compact is arranged. In the process, the second molding material creates a flow component parallel to the joining surface of the first compact on the same surface. As a result, the formation of a high binder concentration portion can be suppressed at the boundary portion (joining portion) between the first compact and the second compact.
According to a second aspect of the invention, there is provided a method of fabricating a metal composite compact wherein, after molding a first compact by the MIM method, a second compact is molded in close contact with a portion of the first compact, after which the first and second compacts are integrated with each other, comprising the steps of:
forming a first cavity by arranging the cavity surfaces of a reference die and a first replacement die in opposed relation to each other and molding the first compact by injection in the first cavity; and
replacing only the first replacement die with a second replacement die having a cavity surface of a different shape while leaving the first compact on the cavity surface of the reference die, thereby forming a second cavity by the first compact and the cavity surface of the second replacement die, and molding the second compact by injection in the second cavity thereby to produce the metal composite compact including the first compact and the second compact integrated with each other.
In the fabrication method according to this aspect of invention, the first cavity is formed first of all by the reference die and the first replacement die. Specifically, two dies making up a first mold member are each formed with a cavity surface, and these two cavity surfaces are placed in opposed relation to each other thereby to form a first cavity of a shape corresponding to the desired first compact. The metal powder for the first compact is injected into the first cavity. The metal powder, as explained with reference to the prior art, is in the form of a mixture with a binder heated to a predetermined temperature and having fluidity.
After molding the first compact in the first cavity, only the first replacement die is replaced by the second replacement die without releasing the first compact. The second replacement die has a cavity surface corresponding to the desired shape of the second compact, and forms a second cavity with the surface of the first compact in opposed relation thereto. The metal powder for the second compact is injected into this second cavity. This metal powder is also in the form of a mixture with a binder heated to a predetermined temperature and having fluidity.
As described above, according to this invention, after molding the first compact by injection, the second compact can be molded by injection on the first compact without removing it from the die. Specifically, immediately after molding the first compact by injection, the second compact can be molded by injection within a short time simply by exchanging the first and second replacement dies with each other. As a result, the second compact can be brought into contact with the first compact left on the cavity surface of the reference die without significantly reducing the temperature of the first compact.
A very satisfactory condition of the boundary portion between the first compact and the second compact can be obtained by suppressing the temperature drop of the first compact.
Specifically, the first compact and the second compact each contain a binder as well as the metal powder making up the main component. In the case where a mixture of the binder and the metal powder is used for injection molding, the fluidity characteristic thereof causes the concentration of the binder in the surface portion to increase. Also, the binder solidifies and loses the fluidity when the temperature drops. In the case where a second compact is molded by injection on the surface of the first compact after the first compact is cooled and solidified, therefore, a boundary layer high in binder concentration may be formed between the first compact and the second compact. If the mold is degreased and sintered with a boundary layer high in binder concentration, the distance between the metal powder of the two compacts is increased excessively, thereby often making it impossible to obtain a satisfactory sintered state.
By maintaining an appropriately high temperature of the first compact as described above, the surface of the first compact can be maintained in a state of some fluidity. Even after the first compact is solidified, on the other hand, the fluidity thereof can be easily restored by the heat transmitted from the second compact. In such a case, therefore, the distance between the metal powder of the first and second compacts can be reduced by maintaining the fluidity of the portion of the boundary therebetween high in binder concentration. Thus, the quality of the sintered compact finally obtained can be improved.
Also, according to this invention, as described above, the fact that the first compact is left on the cavity surface of the reference die can improve the dimensional accuracy of the resulting metal composite compact.
Further, the second compact can be molded by injection substantially without intermission simply by replacing the first replacement die and the second replacement die with each other as described above. Therefore, the metal composite compact can be fabricated with a very high efficiency for a reduced fabrication cost. As described above, according to this invention, a fabrication method is provided in which a metal composite compact including a plurality of compacts of different types of material or the same type of material integrated with each other can be fabricated with a high dimensional accuracy at low cost.